This invention relates to a power semiconductor device and more particularly to a current sensing circuit adapted for a power IC which integrates a control circuit and an output stage element of a high withstand voltage and a heavy current and to a protection circuit for such a power IC.
Conventionally, in the circuits utilizing power semiconductor elements, for preventing the power semiconductor elements from being destroyed by an excess current or a short-circuiting, there is known a method of detecting the current through the power semiconductor element with the use of a shunt resistor through the voltage drop across the resistor. In a power IC integrating a power semiconductor element of a high withstand voltage and a heavy current and a control circuit for controlling the power semiconductor element, however, the above-mentioned method of using a shunt resistor causes a problem of heat generation due to the power loss and hence cannot be employed.
The output stage element of a power IC may be a bipolar transistor of the current drive type, a MOSFET of the voltage drive type, and a composite element co-using a MOS structure and a bipolar structure. When bipolar transistors are connected in parallel, it is generally known that there occurs unbalance of currents flowing through the respective transistors, i.e. the so-called current concentration. A current detecting circuit utilizing this current is such a structure disclosed in U.S. Pat. No. 4,139,181. This structure is reproduced in FIG. 1. In the circuit of FIG. 1, the output stage employs multi-collector PNP transistors 52 and 53. The emitter of the transistor 52 is connected to the positive electrode of a voltage source 13, while the emitter of the transistor 53 is connected to the positive electrode of the voltage source 13 through a resistor 54. The base terminals of the transistors 52 and 53 are applied with the same driving signal from the control circuit 59. Numerals 62 and 63 denote substrate collectors which supply currents to a load 14. Numerals 60 and 61 denote lateral collectors which are connected respectively with MOSFETs 56 and 55 forming a current mirror circuit. The ratio of areas of the MOSFETs 55 and 56 is 1:n.
Next, the operation of the circuit will briefly be described. Letting the current flowing through the lateral collector 61 be I.sub.7, the MOSFET 56 will have a function of possibly flowing a current of nI.sub.7 by the function of the current mirror circuit. Letting the current flowing through the lateral collector 60 be I.sub.8, a relation of nI.sub.7 &gt;I.sub.8 holds when the voltage drop across the resistor 54 is small. The output voltage derived at a point 64 is of low level. Next when the current flowing through the load 14 increases to increase the voltage drop across the resistor 54, the current is more concentrated in the transistor 52 to produce a situation I.sub.8 &gt;nI.sub.7. As a result, the output voltage derived at point 64 changes to a high level.
By the above-described operation, the lateral collector current exceeding a set value is detected and the current flowing through the substrate collectors to the load is indirectly detected to exceed an excess current level.
In contrast to the above-described method, U.S. Pat. No. 4,553,084 proposes a method of utilizing a MOSFET as the output stage element, the MOSFETs being superior in the current balance when connected in parallel.
According to this method, utilizing the fact that the power MOSFET of the output stage element comprises an integrated parallel connection of a multiplicity of small-capacity MOSFETs, which are called cells, a few or several cells used as a detecting transistor and a resistor is connected to this detecting transistor.
When the voltage drop across the resistor connected to the detecting transistor is small, the detecting transistor allows such a detection current to flow which is determined by the cell ratio with respect to the main current flowing through the power MOSFET of the output stage. The voltage produced by this detection current and the above-mentioned resistor is compared with a reference voltage, to detect the main current indirectly.
According to the former prior art utilizing multi-collector PNP transistors, it is difficult to precisely set the reference current I.sub.7 because of the dispersion in the precision of the resistor 54 and of the variation of the resistance by the temperature rise.
Further, although it is required that the currents flowing through the lateral collector and through the substrate collector are in a proportional relation, occurrence of current concentration can occur in a bipolar transistor and the precision of the indirect current detection becomes of problem. Further, the MOSFETs 55 and 56 connected in parallel to the load 14 are both required to have a high withstand voltage, which means being expensive.
According to the latter prior art utilizing the MOSFET as the output stage element, the structure is simple and the precision of detection is good at the room temperature. At the high temperature, the resistance value changes to give influence to the detection precision. Further, when the resistor is integrated in the semiconductor substrate, there are also problems such as the dispersion of resistances due to the manufacturing process.
In a power IC, various other protection functions are required such as assurance of operation of the output stage element in the area of safety operation (ASO), protection from the temperature and the excess voltage, and protection from the arm short-circuiting in the case when the output stage element has an inverter construction, as well as the protection from the excess current. Each of the above-mentioned prior art only applies to the protection function for the excess current, and other circuit means are required to be installed for achieving other protection functions.